[Haskell-cafe] Define combination of type classes?

[Sean Seefried]

On 24/03/2006, at 12:45 PM, Fritz Ruehr wrote:

> What is the easiest way to name a combination of type classes,  
> i.e., to abbreviate the fact that a certain type is an instance of  
> several classes simultaneously? I have a vague sense that this is  
> do-able, but that I am messing up by trying to use an empty class  
> body as below.
>
> So in the code below, I try to use FooBar to abbreviate the  
> conjunction of Foo and Bar. But while f (which uses a FooBar  
> constraint) has a valid definition, it can't be used. On the other  
> hand, g (which uses the long-winded constraint), is both a valid  
> defined and useable.
>
> (In a real example, imagine that FooBar names a conjunction of a  
> half dozen things, so that the g-like form really is onerous,  
> whereas the f-like form would be sweet and tidy :) .)
>

Hi Fritz!

You only need to do a couple of things to get this working. Add an  
instance declaration:

instance (Foo a, Bar a) => FooBar a

But for this to work you need to allow undecidable instances (and - 
fglasgow-exts).

To have this type class synonym trick work you need both the class  
and instance declaration:

class    (Foo a, Bar a) => FooBar a
instance (Foo a, Bar a) => FooBar a

The first ensures that members of class FooBar will inherit the  
methods of classes Foo and Bar. The second ensures that if there is a  
Foo and a Bar instance then there will be a FooBar instance. You were  
lacking this in your code hence the error message:

 > f 'a'

No instance for (FooBar Char)
       arising from use of `f' at <interactive>:1:0
     Probable fix: add an instance declaration for (FooBar Char)
     In the definition of `it': it = f 'a'

This is a neat trick. I've also used it to reduce onerous contexts.

Sean